Both fast examples create a dependence between loop iterations. The ADD+SBB case creates a dependence between the carry flag and the value loaded from memory; t

Both fast examples create a dependence between loop iterations. The ADD+SBB case creates a dependence between the carry flag and the value loaded from memory; the INC + XOR creates a dependence on the value of RDX and the value loaded from memory. I don't know what exactly is the mechanism here, but the data dependencies seem to prevent memory loads to be speculated like crazy, presumably the source of the slowdown. Another data point: replace SUB RCX, 1 / JNZ by LOOP. LOOP is much slower, something like 7 uops, but it still results in a speedup. Here I cannot blame data dependencies. LOOP is microcoded, so perhaps its internal implementation has the same effect.